This invention relates to reactive adducts of certain vinyldioxo compounds and to curable coating compositions comprising such reactive adducts.
Vinyldioxo (VDO) compounds are cyclic acetals and are described in detail in U.S. Pat. Nos. 3,010,918; 3,010,923; and 3,197,484 by Ikeda, U.S. Pat. No. 3,014,924 by Brachman, and an article by S. Hochberg entitled The Chemistry of the Vinyl Cyclic Acetals and Their Air Drying Reactions, 48 Journal of the Oil and Colour Chemists' Association (JOCCA) 1043-68 (1965). The simplest compounds in this class are made by a reaction of acrolein with a compound having two hydroxyl groups, either on adjacent carbon atoms or on carbon atoms separated by an additional carbon atom. When more than two hydroxyl groups are present in a compound, different pairs of hydroxyl groups can react with the aldehyde to form a cyclic acetal. Typical compounds having at least two hydroxyl groups include, for example, ethylene glycol, glycerin, 1,2,6-hexanetriol, and trimethylolpropane. Depending on the number and type of hydroxyl groups, the resulting VDO can be either a substituted 1,3-dioxolane or a substituted 1,3-dioxane, but frequently it is a mixture of a dioxolane with a dioxane. The reaction of acrolein (1) with trimethylolpropane (2) is shown below in Equation 1. The formation of VDO compounds, like other acetal-forming reactions, is catalyzed by acids. ##STR1##
The reaction product according to formula (3) is a 2-vinyl-1,3-dioxane substituted with an ethyl group and a hydroxymethyl (methylol) group in the 5-position. Analogous reactions can be used to prepare substituted rings having four to ten carbon ring members, that is, substituted dioxirane, dioxetane, dioxolane, dioxane, dioxepane, dioxocane, dioxonane, and dioxecane. An improved process for the synthesis of vinyldioxo compounds is disclosed in copending U.S. application Ser. No. 08/435,251, herein incorporated by reference in its entirety.
VDO compounds and their various derivatives have been described as useful polymerizable materials, which have the potential of providing both pigmented and clear-coat finishes in automotive and other applications. Those compounds polymerize in the presence of oxygen, such reactions being catalyzed by cobalt compounds. In an aqueous medium and in the presence of acids, the VDO compounds are unstable, so that further reactions are carried out in either a neutral, alkaline, or organic medium. In spite of the great industrial potential of VDO compounds, they have not been successfully commercialized in high performance coatings.
It has been found that VDO compounds can be polymerized, with ring opening, in the presence of an acid. For example, one of several reactions that take place involves the above compound of formula (3) in the following equation 2: ##STR2##
The hydroxyl group attached to the repeating group of the VDO polymer in formula (4) can be used for crosslinking by reaction with a difunctional or polyfunctional compound to form a solid coating. The VDO polymer crosslinking reaction typically comprises one or two liquid components in the absence of a solvent or with a minimum amount of a solvent or diluent.
The copending application designated Ser. No. 08/435,919, herein incorporated by reference in its entirety, discloses a class of adducts formed by reacting VDO compounds with difunctional compounds having two reactive functional groups, at least one of those functional groups being a silyl group, most preferably a di- or trialkoxysilyl group where the alkyl groups have from 1 to 4 carbon atoms, those adducts being represented by the following generic formula (5): ##STR3## where R is an alkyl having 1 to 12, preferably 1 to 6, carbon atoms, most preferably methyl or ethyl, or R is a branched or cyclic alkyl having 3 to 12, preferably 5 or 6, carbon atoms, and Z is a divalent radical between the oxygen on the VDO moiety and the terminal silyl group.
This invention discloses a class of VDO adducts that display hybrid crosslinkng when used in coating formulations. In particular, VDO compounds as disclosed above are reacted with various cyclic reactants to form VDO adducts. During the formation of the VDO adducts, the cyclic reactants undergo ring opening and the VDO ring structure remains intact. The resultant VDO adducts have two mechanisms by which they can be crosslinked: the VDO ring structure and reactive groups derived from the cyclic compounds that underwent ring opening.
Hybrid crosslinking occurs when the VDO adducts are used in coating formulations in the presence of acid, a traditional crosslinking agent, and optionally an additional catalyst for the crosslinking agent, such that two or more different types of crosslinking reactions occur simultaneously, subsequently, or both. The acid polymerizes the VDO adduct by opening the VDO ring structure. The traditional crosslinking agents react with the reactive groups derived from the cyclic compounds that underwent ring opening. As shown in the examples, the hybrid crosslinking provides superior results when used in coating formulations.